In the various types of data recording systems based on use of a magnetic medium, a magnetic transducer head is used to create the magnetic patterns in the medium and to read back the data after recording it. The typical magnetic head has an O-shaped core element which provides a magnetic flux path including a flux gap across which flux fringing occurs. During reading and writing, the flux gap is placed in close proximity with the medium, and the medium is moved with respect to the gap. The fringing flux during writing enters the medium, causing the magnetic patterns to be created therein. The flux flow for the writing process is created by a winding through which the flux path passes.
During reading, the medium is again moved relative to the gap, and the magnetic pattern in the medium causes flow of flux in the flux path, which flux flow recreates the magnetic pattern in the medium. By detecting, i.e. converting it to an electrical signal, this flux flow can be converted into the original data. During reading, voltage across this same winding can be used to detect the flux flow, or a separate flux-detecting element such as a magneto-resistive element can be used.
There are two types of magnetic recording heads now in general use. The older type has a ferrite or other core providing the flux path and a separate wire winding for writing and reading. The newer is formed by photolithographic processes and is typically referred to as a thin film recording head. The invention forming the subject matter here is concerned with the ferrite core type of recording head.
The complete O core of a ferrite core head typically comprises a C core element, so called because it is in the shape of a C, and an I core element having an elongated rectangle shape. The I core and C core elements are attached to each other in some manner with the I core element connecting the arms of the C core element to thereby close the flux path. One of the connecting points between these two elements forms the flux gap. Typically, the flux gap is formed of a hard non-magnetic material such as glass or alumina. The two facing surfaces of the I core and the C core which define the flux gap are called the gap faces.
To make the individual cores used for the heads, a C bar element whose cross section is uniformly identical to that of an individual C core element, is bonded to an I bar element whose cross section is uniformly identical to that of the individual I core elements. Usually the I and C bars are from 50 to 100 times as long as the finished core width, with the gap width substantially smaller than the core width. A thin layer of gap material is placed, now usually by a sputtering process, at the interface corresponding to the gap of the finished cores. The I bar is then clamped to the C bar to close its ends, in combination with it assuming, if one uses his or her imagination, the shape of an O bar. (Hereafter, a C bar with an I bar juxtaposed in the position to form an O bar, but before these bars are bonded to each other, will be referred to as an unbonded O bar.) The unbonded O bar is then heated to a temperature sufficient to fuse the gap material to the adjacent I or C bar gap face, fixing the gap length and bonding the bars to each other in the O bar shape. (By gap length is meant the spacing of the I bar from the C bar, and is by analogy to the direction of movement of the magnetic medium to the transducer heads of which the O bar will become a part.) The bonded O bar resulting is then sawed transversely into individual cores which can be wound and mounted in a suitable support. For use in rigid disk drives, these cores are mounted in slots in hard ceramic sliders which are designed to aerodynamically fly in close proximity to the disk surfaces.
The individual O bars are quite small, with a typical example being perhaps one inch long and less than a tenth of an inch in the width and height dimensions.
It is necessary to accurately control the gap length for each individual core which will be eventually sawed from an O bar, since gap length interrelates with the other core dimensions and parameters of the medium as well and deviations from the design gap length can adversely affect performance. Control of gap length has always been a difficult problem because of the small dimension involved, on the order of 20.mu.inches (about 0.5.mu.) long, relative to the length of a typical O bar, and reliably reproducing such a small dimension in a manufacturing process is very difficult. The tolerances are now typically required to be .+-.5.mu.in. (.+-.0.1.mu.). While these tolerances can be achieved with known processes, the yield is not as good as is desired, and obviously, there is no way to correct defective gap lengths in individual cores.
There are a number of different ways in which the related problems of accurate gap formation and uniform I bar to C bar bonding may be effected in a manufacturing process. However, it is difficult to cause the individual surfaces of a pair of ferrite bars to accurately contact each other along the entire length of their bonding interface, and if there is not this intimate contact the bond is faulty in at least one of its mechanical and magnetic properties. A solution to this problem is provided in IEEE Transactions on Magnetics, Vol. MAG-20, No. 5, September 1984, pp. 1503-1505 by Rigby, "Diffusion Bonding of NiZn Ferrite and Nonmagnetic Materials", where a what will be hereafter referred to as a "granule vise" is described. Such a vise employs hard, heat resistant granules contained in a cylindrical cavity. Pairs of I and C ferrite bars with their surfaces to be bonded properly juxtaposed to form unbonded O bars are embedded in the granules. Then powerfully compressing the granules with a piston or die driven into the cavity strongly compresses the I and C bars against each other. Applying heat to the unbonded O bars while thus compressing them against each other results in high quality diffusion bonding along the entire length of the interface where the I and C bars contact each other.
To assure a strong mechanical bond between the I and C bars and between the I and C cores after the individual cores are cut from the bonded O bars, it is the usual practice to place a small bonding glass rod in the interior opening of each unbonded O bar adjacent the edge of the flux gap where it is bordered by the interior opening. The heat of the bonding operation melts the glass, causing it to form a glass fillet adjacent to the interior edge of the flux gap. By orienting the O bars with the interior edges of their flux gaps below the remainder of the O bars' interior openings, the glass will flow under the force of gravity into the space adjacent the interior edge of the flux gap thereby firmly attaching the I bars to the C bars, and, after the sawing operation, the individual I and C elements of the cores to each other.